1. Field of the Invention
The present invention relates to a torque sensor for detecting torque applied to a torsion bar to be used in a rotating force transmission system, in particular, in an electric power steering system.
2. Description of Related Art
Conventionally, according to a device disclosed in JP-A-8-159887 for detecting torsion torque applied to a torsion bar, a magnet and a magnetic sensor are used. The magnet is fixed to an axial end of the torsion bar and the magnetic sensor is fixed to the other axial end of the torsion bar. When the torsion torque is applied to the opposite axial ends of the torsion bar, the torsion bar is twisted so that a rotation displacement of the magnetic sensor relative to the magnet is changed. Accordingly, an output responsive to the applied torque is generated from the magnetic sensor.
According to the detecting device mentioned above, electric contacts such as a brush and a slip ring for supplying electric power to and picking up a signal from the magnetic sensor are necessary, since the magnet and the magnetic sensor are fixed to the opposite axial ends of the torsion bar that is rotated. The use of the brush and the slip ring is prone to deteriorate reliability of the detecting device.
Further, according to another detecting device disclosed in JP-A-6-281513, though it is similar to JP-A-8-159887 in that the magnet and the magnetic sensor are used, helical gears, to which the magnet is fixed, are used for converting the rotation displacement of the axial end of the torsion bar relative to the other axial end of the torsion bar into an axial displacement of the magnet relative to the magnetic sensor that is fixed to a housing. Accordingly, the electric contacts for supplying electric power to and picking up a signal from the magnetic sensor are not necessary.
However, this detecting device uses the gears so that construction of the detecting device is complicated. Further, the device has a drawback in performance since detection errors and response delays seem to be unavoidable due to backrush of the gears and possible wear of the gears.